In 3GPP (3rd Generation Partnership Project), which is a project aiming to standardize a mobile communication system, introduction of a dual connectivity scheme (Dual connectivity) in Release 12 and thereafter is planned (see Non Patent Document 1). In the dual connectivity scheme, a user terminal simultaneously establishes connections with a plurality of base stations. To the user terminal, a radio resource is assigned from each base station, and thus, it is possible to expect an improvement in throughput.
In the dual connectivity scheme, of a plurality of base stations that establish a connection with a user terminal, one base station (hereinafter, referred to as “master base station”) only establishes an RRC connection with the user terminal. On the other hand, of the plurality of base stations, another base station (hereinafter, called “secondary base station”) provides an additional radio resource to the user terminal without establishing an RRC connection with the user terminal. It is noted that the dual connectivity scheme may also be called an inter-base station carrier aggregation (inter-eNB CA).